The object of the invention is a method for correcting an error due to variations in the sample volume in a liquid scintillation counter provided with a cylinder shaped optics. Such error is produced due to the dependence of the counting efficiency on the sample volume. The sample which is formed by dissolving a substance to be analyzed in a scintillation liquid and placing the dissolved substance into a transparent or translucent sample container inserted into a vertically positioned cylinder shaped counting chamber provided with photomultiplier tubes disposed on the opposite ends of the chamber and operating in coincidence.
Liquid scintillation counters are commonly used for counting samples which contain low energy beta or corresponding particles emitting radioactive isotopes such as tritium and carbon-14. The range of the low energy beta particles in the sample is generally, at the most, a few tens of micrometers. As a consequence, the specimen to be analyzed has to be dissolved into a scintillation liquid, in which the molecules of the isotope to be counted are close enough to the molecules of the scintillation substance so that the beta particles emitted by the isotope to be counted can interact with the molecules of the scintillation substance. In this interaction process, a part of the energy of the beta particle is transformed into light, which is converted to an electric pulse generally by means of two photomultiplier tubes which operate in coincidence. The purpose of the coincidence operation is the elimination of thermal noise of the photomultiplier tubes. The amplitude of the electric pulse is proportional to the energy of the beta particle interacted with the scintillation substance.
Because the energies of the emitted beta particles are distributed in a way characteristic of the beta decay of the isotope to be counted, a continuous spectrum corresponding to the energy distribution of the emitted beta particles is obtained by means of a multichannel analyzer incorporated in the counter. This continuous spectrum has certain characteristic properties e.g. total counts, number of counts in a certain "counting window" or channel region of the multichannel analyzer, end point, maximum value and center of mass, i.e. the centroid of the obtained spectrum. It can be determined in which channel of the multichannel analyzer the end point, the maximum value and the center of mass are located, i.e. the channel coordinates of these values can be determined.
A liquid scintillation counter provided with cylinder shaped optics is defined as a liquid scintillation counter in which the transparent or translucent sample container containing the specimen to be analyzed and the scintillation liquid is inserted in a vertically positioned cylindrical counting chamber with both ends open. The inner surface of the counting chamber itself is made of, or the surface is coated by, a light reflecting or scattering material. The purpose of the light reflecting or scattering inner surface of the counting chamber is to guide the scintillation photons produced by the absorption of the beta particle in the scintillation substance to the photomultiplier tube photocathodes placed at both open ends of the cylinder shaped counting chamber.
The counting efficiency of a liquid scintillation counter means denotes the probability of the counting system to detect the beta particles emitted by the sample to be analyzed. It has been observed in performed experiments that the counting efficiency of a conventional liquid scintillation counter as well as that of the liquid scintillation counter provided with cylinder shaped optics is dependent on the total volume of the specimen and the scintillation liquid in the sample container. As a consequence, to reach comparable results, the total volumes of the samples to be analyzed should always be same and in the separate containers exactly equal.
Because it is practically impossible to keep the sample volumes always equal, there is always an error in the observed count rate when the volume of the sample deviates from an optimal value. This is due to the fact that the light collecting efficiency of the cylinder shaped optics depends on the optical system formed by the sample, sample container, optics and the photocathodes of the photomultiplier tubes.